Problem 26 - Chemistry: The Central Science

Question

Write balanced equations for each of the following reactions (some of these are analogous to reactions shown in the chapter). (a) Aluminum metal rcacts with acids to form hydrogen gas. (b) Steam reacts with magnesium metal to give magnesium oxide and hydrogen. (c) Manganese(IV) oxide is reduced to manganesc(II) oxide by hydrogen gas. (d) Calcium hydride reacts with water to generate hydrogen gas.

Step-by-Step Solution

Answer

The balanced equations for the reactions described are: (a) \[2Al + 6HCl -> 2AlCl₃ + 3H₂\] (b) \[Mg + H₂O -> MgO + H₂\] (c) \[MnO₂ + 2H₂ -> MnO + 2H₂O\] (d) \[CaH₂ + 2H₂O -> Ca(OH)₂ + 2H₂\]

Step 1: Determine the Reactants and Products.

The reactants are aluminum metal (Al) and an acid (H⁺). Since the acid forms hydrogen gas (H₂), let’s use hydrochloric acid (HCl) as an example. The products will be a salt with aluminum and chloride ions and hydrogen gas (H₂).

Step 2: Write the Unbalanced Equation.

Unbalanced equation: \[Al + HCl -> AlCl₃ + H₂\]

Step 3: Balance the Equation.

By balancing, we have: \[2Al + 6HCl -> 2AlCl₃ + 3H₂\] #b# Steam reacts with magnesium metal to give magnesium oxide and hydrogen.

Step 4: Determine the Reactants and Products.

The reactants are magnesium metal (Mg) and steam (H₂O). The products are magnesium oxide (MgO) and hydrogen gas (H₂).

Step 5: Write the Unbalanced Equation.

Unbalanced equation: \[Mg + H₂O -> MgO + H₂\]

Step 6: Balance the Equation.

By balancing, we have: \[Mg + H₂O -> MgO + H₂\] This equation is already balanced. #c# Manganese(IV) oxide is reduced to manganese(II) oxide by hydrogen gas.

Step 7: Determine the Reactants and Products.

The reactants are manganese(IV) oxide (MnO₂) and hydrogen gas (H₂). The product will be manganese(II) oxide (MnO) and water (H₂O).

Step 8: Write the Unbalanced Equation.

Unbalanced equation: \[MnO₂ + H₂ -> MnO + H₂O\]

Step 9: Balance the Equation.

By balancing, we have: \[MnO₂ + 2H₂ -> MnO + 2H₂O\] #d# Calcium hydride reacts with water to generate hydrogen gas.

Step 10: Determine the Reactants and Products.

The reactants are calcium hydride (CaH₂) and water (H₂O). The products are calcium hydroxide (Ca(OH)₂) and hydrogen gas (H₂).

Step 11: Write the Unbalanced Equation.

Unbalanced equation: \[CaH₂ + H₂O -> Ca(OH)₂ + H₂\]

Step 12: Balance the Equation.

By balancing, we have: \[CaH₂ + 2H₂O -> Ca(OH)₂ + 2H₂\]

Key Concepts

Chemical Reactions

Chemical reactions are processes in which the identity of substances changes as they break and form bonds to create new compounds.

In the context of the exercise provided, we see various instances of chemical reactions. For example, aluminum metal reacting with hydrochloric acid to form aluminum chloride and hydrogen gas is a typical single displacement reaction. During this process, aluminum displaces hydrogen from hydrochloric acid, showcasing a transformation of substances and the creation of new chemical entities.

Understanding the intricacies of these reactions requires keen observation of the reactants (starting substances) and products (substances formed). The reactants undergo changes due to breaking and formation of new bonds, signaling a completed chemical reaction when the products are formed. This transformation conserves mass according to the Law of Conservation of Mass, which states that matter cannot be created or destroyed in a closed system.

Stoichiometry

Stoichiometry is the branch of chemistry concerned with the quantities of substances that are consumed and produced in chemical reactions. The key principle in stoichiometry is that the conservation of atoms must be maintained, as they are simply rearranged, not created or destroyed.

In the exercise, stoichiometry plays a critical role as it instructs us on balancing chemical equations. To balance an equation such as \(2Al + 6HCl -> 2AlCl_3 + 3H_2\), we must ensure that the number of atoms of each element is the same on both sides of the reaction arrow. This representative equation embodies the stoichiometric relationship that for every 2 moles of aluminum and 6 moles of hydrochloric acid reacted, 2 moles of aluminum chloride and 3 moles of hydrogen gas are produced.

Stoichiometry is not only about balancing equations but also pertains to the quantitative relationship between reactants and products. This could include calculations of reactant molarity, product yield, or even energy changes during the reaction, making it a cornerstone of chemical practice and industry.

Redox Reactions

Redox reactions, short for 'reduction-oxidation reactions,' are a type of chemical reaction in which electrons are transferred between substances. They involve the process where one substance gets oxidized (loses electrons) while another gets reduced (gains electrons).

In the case of the manganese(IV) oxide being reduced to manganese(II) oxide by hydrogen gas, captured in the equation \(MnO_2 + 2H_2 -> MnO + 2H_2O\), manganese is reduced from a +4 oxidation state to a +2 oxidation state, showing reduction. Hydrogen, on the other hand, is oxidized as it goes from an elemental state (0 oxidation state) to being part of water (with an oxidation state of +1).

Understanding redox reactions is vital for many applications, including but not limited to, energy storage (batteries), metallurgy (extraction of metals), and biological systems (cellular respiration). Each redox reaction is made up of two half-reactions: one that depicts oxidation and another showing reduction. Learning how to identify these half-reactions and balance them is key to mastering redox chemistry.